In many industrial and environmental control systems reservoirs of material are used as a tool of flow control, typically to match a time variable input flow to a constant output flow, or the reverse. The systems are seen in wastewater treatment systems, freshwater reservoirs, oil tank farms, plastic feedstocks, fuel bunkers, even in grain silos. In large industrial processes the flow control system often includes pumping or lifting machinery driven by large electric motors. The electrical draw of these motors is significant.
In recent times public utilities have found that installing new electrical generating capacity to handle increased peak demand is expensive and difficult. Therefore many utilities have adopted variable pricing policies to encourage their customers to consume electricity at off-peak hours, thus evening out the demand for electricity during the day. In addition, for many years electric utilities have based certain charges on the peak use by a customer, even if the average draw is far below that of some transient peak demand condition.
Although reservoir systems may be used for storage of any liquid, or liquid like substance including viscous resins, slurry, or bulk granular material, the typical system to which this invention applies is an urban sewage drainage system. Such systems usually include a number of reservoirs, or wet wells, having inlets through which sewage and surface run-off flow are received. The rate of this inflow varies depending on factors such as the time of day, and the amount and time of rainfall or snow melt. These wells also have outlets connecting them via a common manifold to a number of pumps whose purpose is to pump out the well and thereby transfer the waste water to downstream pumping stations or processing plants. In other types of system the lifting function of pumps is performed by such lifting means as augurs or conveyor systems. These lifting or pumping systems are often adapted to cope with widely varying inflow conditions. To this end variable speed pumps have been used. A more common approach has been to use a number of less expensive constant speed pumps but to use only one or a few pumps at periods of low inflow, and to activate a larger number of pumps at periods of greater inflow. At the periods of greatest inflow, such as during a heavy thunderstorm, the installed pump capacity may be insufficient to cope with the sudden surge. In those cases the well is provided with an overflow system.
Several features of these systems are known. It is known to bring only one additional electrical motor on line at a time to reduce power surges due to starting transients. It is also known to provide a means of circulating liquid in the sump of the well to prevent the accumulation of solids that have settled in periods of low flow. It is equally known to control the number of pumps used by sensing the liquid level in the well or by measuring the inflow and outflow, typically by pressure sensing or weir measurements.
It is known to control such multiple electrical motor systems with microprocessors. The microprocessor will accept a variety of input signals, such as the depth of liquid in the well, inputs from rainfall sensing stations, or the back pressure in various pipes. It may also accept signals keyed in manually by an operator either to define pump `On` and `Off` depths in the well, or wells, or to override the automatic pump commands. Typically the microprocessor will also obtain information about the system from non-volatile memory. This information might include pressure versus flowrate information about the pumping system, or it might include historic rainfall versus input flowrate information. Based on programs programmed into memory the microprocessor will send signals to activate or deactivate power relays controlling the pump motors.
Most pumping systems will record the number of hours of use of each pump. Various strategies have been adopted to sequence pump usage evenly among the pumps. For example, the first pump turned on may be the one with the lowest number of accumulated hours use, the second one being the next lowest, and so on. Another method is to cycle use among the pumps on a preset sequence.